Every single application has localized files in US English, and a number of other languages.
What you should leave is English, AND if you know you might want to use your Mac ever in any other language, those languages as well. If you don't know a single word of Korean, Polish, Finnish, traditional Chinese, Dutch (and about 15 other included languages) then there is zero reason to keep those localizations. If you use your Mac only in one language that is not English, keep that language and English.